1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for deghosting seismic data collected with one or more streamers having variable depths.
2. Discussion of the Background
During the past years, interest in developing new oil and gas production fields has dramatically increased. However, the availability of land-based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel. Offshore drilling is an expensive process. Thus, those engaged in such a costly undertaking invest substantially in geophysical surveys in order to more accurately decide where to drill or not (to avoid a dry well).
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 tows plural detectors 12. The plural detectors 12 are disposed along a cable 14. Cable 14 together with its corresponding detectors 12 are sometimes referred to, by those skilled in the art, as a streamer 16. The vessel 10 may tow plural streamers 16 at the same time. The streamers may be disposed horizontally, i.e., lying at a constant depth z1 relative to the surface 18 of the ocean. Also, the plural streamers 16 may form a constant angle (i.e., the streamers may be slanted) with respect to the surface of the ocean as disclosed in U.S. Pat. No. 4,992,992, the entire content of which is incorporated herein by reference. FIG. 2 shows such a configuration in which all the detectors 12 are distributed along a slanted straight line 14 that makes a constant angle α with a reference horizontal line 30.
With reference to FIG. 1, the vessel 10 also tows a sound source 20 configured to generate an acoustic wave 22a. The acoustic wave 22a propagates downward and penetrates the seafloor 24, eventually being reflected by a reflecting structure 26 (reflector R). The reflected acoustic wave 22b propagates upward and is detected by detector 12. For simplicity, FIG. 1 shows only two paths 22a corresponding to the acoustic wave. However, the acoustic wave emitted by the source 20 may be substantially a spherical wave, e.g., it propagates in all directions starting from the source 20. Parts of the reflected acoustic wave 22b (primary) are recorded by the various detectors 12 (the recorded signals are called traces) while parts of the reflected wave 22c pass the detectors 12 and arrive at the water surface 18. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for the acoustic waves), the reflected wave 22c is reflected back toward the detector 12 as shown by wave 22d in FIG. 1. Wave 22d is normally referred to as a ghost wave because this wave is due to a spurious reflection. The ghosts are also recorded by the detector 12, but with a reverse polarity and a time lag relative to the primary wave 22b. The degenerative effect that the ghost arrival has on seismic bandwidth and resolution is known. In essence, interference between primary and ghost arrivals causes notches, or gaps, in the frequency content recorded by the detectors.
The traces may be used to determine the subsurface (i.e., earth structure below surface 24) and to determine the position and presence of reflectors 26. However, the ghosts disturb the accuracy of the final image of the subsurface and, for at least this reason, various methods exist for removing the ghosts, i.e., deghosting, from the results of a seismic analysis.
U.S. Pat. Nos. 4,353,121 and 4,992,992, the entire contents of which are incorporated herein by reference, describe processing procedures that allow ghosts to be removed from recorded seismic data by using an acquisition device that includes a seismic streamer slanted at an angle (on the order of 2 degrees) to the surface of the water (slanted streamer).
Using slanted streamers, it is possible to achieve ghost suppression during the data summation operation (during pre-stack operations). In fact, the acquired data is redundant, and the processing procedure includes a summation step or “stacking” for obtaining the final image of the subsurface structure from the redundant data. Ghost suppression is performed in the art during the stacking step because the recordings that contribute to the stack, having been recorded by different receivers, have notches at different frequencies, such that the information that is missing due to the presence of a notch on one seismic receiver is obtained from another receiver.
Further, U.S. Pat. No. 4,353,121 describes a seismic data processing procedure based on the following known steps: (1) common depth point collection, (2) one-dimensional (1D) extrapolation onto a horizontal surface, or “datuming,” (3) Normal MoveOut (NMO) correction, and (4) summation or stack.
Datuming is a processing procedure in which data from N seismic detectors Dn (with positions (xn, zn), where n=1, . . . N and N is a natural number, xi=xj but zi is different from zj with i and j taking values between 1 and N), is used to synthesize data corresponding to seismic detectors that have the same horizontal positions xn and the same constant reference depth z0 for all the seismic detectors.
Datuming is called 1D if it is assumed that the seismic waves propagate vertically. In that case, the procedure includes applying to each time-domain recording acquired by a given seismic detector a delay or a static shift corresponding to the vertical propagation time between the true depth zn of a detector Dn and the reference depth z0.
Similar to U.S. Pat. No. 4,353,121, U.S. Pat. No. 4,992,992 proposes to reconstitute from seismic data recorded with a slanted cable seismic data as would have been recorded by a horizontal cable. However, U.S. Pat. No. 4,992,992 takes into account the non-vertical propagation of the seismic waves by replacing the 1D datuming step of U.S. Pat. No. 4,353,121 with a 2D datuming step. The 2D datuming step takes into account the fact that the propagation of the waves is not necessarily vertical, unlike what is assumed to be the case in the 1D datuming step proposed by U.S. Pat. No. 4,353,121.
The methods described in U.S. Pat. Nos. 4,353,121 and 4,992,992 are seismic processing procedures in one dimension and in two dimensions. Such procedures, however, cannot be generalized to three dimensions. This is so because a sampling interval of the sensors in the third dimension is given by the separation between the streamers, on the order of 150 m, which is much larger than the sampling interval of the sensors along the streamers, which is on the order of 12.5 m. Also, existing procedures may apply a deghosting step at the beginning of the processing, which is not always very efficient.
Thus, the above-discussed methods are not appropriate for seismic data collected with streamers having a curved profile as illustrated in FIG. 3. Such configuration has a streamer 52 with a curved profile defined by three parametric quantities, z0, s0 and hc. It is noted that not the entire streamer has to have the curved profile. The first parameter z0 indicates the depth of the first detector 54a relative to the surface 58 of the water. The second parameter s0 is related to the slope of the initial part of the streamer 52 relative to a horizontal line 64. The example shown in FIG. 3 has the initial slope s0 equal to substantially 3 percent. It is noted that the profile of the streamer 52 in FIG. 3 is not drawn to scale because a slope of 3 percent is a relatively small quantity. The third parameter hc indicates a horizontal length (distance along the X axis in FIG. 3 measured from the first detector 54a) of the curved portion of the streamer. This parameter may be in the range of hundreds to thousands of meters.
For such streamers, a deghosting process has been disclosed in U.S. patent application Ser. No. 13/272,428 (herein '428) authored by R Soubaras, the entire content of which is incorporated herein. According to '428, a method for deghosting uses joint deconvolution for migration and mirror migration images for generating a final image of a subsurface. The deghosting is performed at the end of the processing (during an imaging phase) and not at the beginning as with traditional methods. Further, '428 discloses that no datuming step is performed on the data.
However, the existing methods need a velocity field in order to achieve the deghosting. Thus, if there is a case in which the velocity field is not available, there is a need for a method that is capable, particularly for pre-processing steps such as velocity picking or demultiple, to deghost the data without knowledge of the velocity field. Accordingly, it would be desirable to provide systems and methods that have such capabilities.